New Book: Ultrasonic Methods of Non-destructive Testing
By Jack Blitz and Geoff Simpson
First edition 1996
Non-Destructive Evaluation Series
234 x 156 mm, 280 pp, Hardback.
ISBN 0 412 60470 1
Published by Chapman & Hall London Home page
Our comment:
A new UT book that lets your eyes rest from watching the computer screen. You could also take it everywhere you go. The literature is written to be easy to read and provides a good mix between theory and practice.
Rolf Diederichs
Preface
This book is intended to provide an account of the theory and practice
of everyday ultrasonic non-destructive testing although references are
made to relevant latest developments, some of which are still in the
research stage. It is aimed to cater for a wide range of readers extending
from those embarking on associate degrees or national certificate
courses to undergraduate and postgraduate students, researchers and
operators, engineers and management involved with non-destructive
testing.
In the preparation of this work, the authors are indebted to Professor
D.D. Imrie, Dean of the Faculty of Science, at Brunel University and
Mr C.D. Wells of Messrs. Wells-Krautkr�mer for the use of their
facilities and to Mr M. Berke of Krautkr�mer GmbH and Co., Commander G.M. Selous OBE and his colleagues at CNS Electronics Ltd,
Dr E.L. Short of the Department of Chemistry at Brunel University and
Dr M.G. Silk of the National Nondestructive Testing Centre at Harwell
for their kind help.
Jack Blitz, Department of Physics, Brunel University, UK.
Geoff Simpson, Letchworth, UK.
CHAPTER 1 - Introduction
GENERAL CONSIDERATIONS
Ultrasonics is the name given to the study and application of
ultrasound,
which is sound of a pitch too high to be detected by the human ear, i.e.
of frequencies greater than about 18 kHz. Ultrasonic waves have a wide
variety of applications over an extended range of intensity, including
cutting, cleaning and the destruction of tissue at the upper extremity
and
non-destructive testing (NDT) at the lower end. A non-destructive
test is one in which there is no impairment of the properties and
performance in future use of the object under examination. With
ultrasonic non-destructive testing, which is effectively a mechanical
method, periodic mechanical stresses are applied to the object. It is
essential that there have been no changes in dimensions and structure
of the object when the test is completed. This can only be achieved
when the maximum applied stresses do not exceed the elastic limit
below which Hookels law is obeyed, so that the resultant strain is
proportional to the applied stress. Hence it is necessary that the ultra-
sonic intensity is sufficiently low for the elastic limit not to be exceeded.
Ultrasonic testing consists effectively of the propagation of low amplitude waves through a material to measure either or both the time of
travel and any change of intensity for a given distance. Applications
include distance gauging, flaw detection and measuring parameters
(such as elastic moduli and grain size) which are related to the material
structure. Reasons for using ultrasonic as opposed to audible fre
quencies include the following.
- Wavelengths decrease inversely with frequency. This is advant
ageous with the testing of smaller samples having dimensions of the
same order of magnitude as the wavelength used. The use of short
wavelengths enables the employment of shorter pulses, thus pro
viding higher degrees of resolution for defect detection. Furthermore,
the degree of beam spread decreases with rise in frequency and,
hence, an increase in directivity; this is of great importance for
locating defects.
- Attenuation generally increases with frequency with the result that
the degree of attenuation, which is often related to material structure,
can be more easily detected and rneasured. In some instances, such
as the testing of highly attenuating Materials such as concrete with
the resonance and damping capacity method, satisfactory results can
be obtained with the use of higher audible frequencies, e.g. l0kHz.
The application of ultrasound to non-destructive testing was first made
possible by the discovery of the piezoelectric effect by the brothers
Pierre and Jacques Curie in 1880. However, it was not until World War
II when the necessary technology became available, after the discovery
of radar, that any effective progress was possible. It was then that
Firestone in the USA and Sproule in the UK, working independently of
one another, developed the pulse-echo ultrasonic flaw detector. Since
then, ultrasonic testing has progressed at a remarkable rate as a result
of rapid advancements in electronics and in computer technology. It is
now more frequently used than radiology for non-destructive testing.
The ultrasonic method of non-destructive testing is currently the
most widely used of the testing methods, disregarding the obvious
ones of looking, feeling, measuring and weighing. These 'obvious'
tests should, where possible, be conducted initially in all cases. It is a
sad fact that much time and money have been wasted by conducting
tests which reveal defects that could have been observed by the naked
eye. The other principal methods are:
radiology, e.g. the uses of X- and -y-rays and of neutron beams (for
example, Halmshaw 1982);
electrical and magnetic methods, e.g. magnetic particle inspection
and other magnetic flux leakage methods, eddy current testing,
potential drop and AC field methods and microwave testing (for
example, Blitz 1991); and
liquid penetrant testing (for example, McMaster 1982).
Of the commonly-used methods, only ultrasonic and radiological
techniques can detect internal flaws with a very high degree of certainty
although, in limited cases, detection may be possible in dielectric
Materials with microwaves and in ferromagnetic Materials with
magnetic flux leakage methods using highly sensitive Hall element
detectors. The use of the other methods is mainly restricted to detecting
surface and sub-surface defects for which ultrasonic testing is often
successful, but it should be borne in mind that these other methods,
especially the magnetic particle and dye penetrant methods can be
performed more speedily. However, the latter techniques cannot
provide reliable, if any, indications of crack depth which can be accurately measured with ultrasound, eddy currents and AC potential
drop and field measurements. For metals, the use of eddy currents is
preferred for evaluating shallow surface cracks in metals and, although
the AC measurements may be used for deeper crack detection and
measurements, the ultrasonic technique has the advantage of accurately
locating crack tips and thus providing indications of the angles of
cracks.
The main advantages of ultrasonic testing are:
- testing can be carried out from a single surface;
- a high degree of penetration is possible in many commonly-used
materials, which is in contrast with the lower degree of penetration
encountered with radiological testing of metals;
- accuracy in locating and measuring defects;
- the ability to detect and size very small defects; and
- compatibility with automatic scanning devices and with micro-
processors and computers.
The principal drawbacks are these:
- Operators must be properly trained, highly experienced and possess
a high degree of reliability and integrity. However, with 100 %
automated testing, these requirements may be somewhat relaxed
provided that the functions of the automated system are thoroughly
understood.
- With manual operation over a large surface area only a small part of
a surface can be scanned at a time, although this can be improved
upon, where feasible, with the use of transducer arrays.
- A high degree of coupling between the transducer and the surface to be scanned is required, though much progress is now being made
with the development of non-contact transducers.
This book is intended to provide an account of the theory and practice
of everyday ultrasonic testing, although references are made to relevant
latest developments, some of which are still in the research stage. It
is aimed to cater for a wide range of reader extending from those
embarking on associate degree or national certificate and BTEC courses
to postgraduate researchers and, of course, qualified engineers and
NDT personnel. To allow for this, the mathematics are kept at as low a
level as possible in the main parts of the text and derivations of the
relatively more complex equations are located in the Appendices.
Following normal practice, SI units are used in this book and readers
who are not too familiar with them are referred to Appendix H.
REFERENCES
- Blitz, J. (1991) Electrical and Magnetic Methods of Nondestructive Testing. Adam
Hilger, Bristol.
- Halmshaw, R. (1995) Industrial Radiology - Theory and Practice, 2nd edn
Chapman & Hall, London.
- MC Master, R.C. (ed) (1982) Nondestructive Testing Handbook, 2nd edn, Vol 2 (Liquit Penetrants). American Society for Nondestructive Testing, Columbos, OH.
Table of Contents:
- Introduction.General considerations. References. The
propagation of low amplitude ultrasound. Introduction. Free and forced
vibrations. Wave motion. Attenuation of plane waves. Reflection and
transmission of plane waves. Stationary waves. Diffraction. Continuous
and
pulsed waves. Phase and group velocities. Focusing systems. References.
Ultrasonic characterization. General considerations. Ultrasonic speeds.
Ultrasonic scattering and absorption. Anisotropic materials. Surface and
lamb
waves. References.
Ultrasonic transducersIntroduction. Piezoelectric transducers:
principles.
The properties of piezoelectric transducers. Piezoelectric ultrasonic
probes.
Other transducers. References. The principles of ultrasonic testing.
Introduction. Pulse methods. The pulse-echo method. The
pulse-transmission (or
shadow) method. Scanning and imaging. Continuous wave techniques. The
ultrasonic goniometer. Impedance measurements. Surface and lamb wave
measurements. Acoustic emission testing. References.
Ultrasonic testing equipment. Introduction. High-frequency pulse
operating
equipment. Ultasonic thickness gauges. Miscellaneous applications.
References.
Ultrasonic Flaw Detection.Introduction. Calibration of equipment. Angle
probe
operation. The testing of forgings and of rolled and wrought metals.
Immersion
testing techniques. References. Flaw sizing in metals. Introduction.
Probe
movement sizing methods. Amplitude methods. Attenuation corrections.
References. The testing of metals. General considerations. Rolled
products.
Rods and bars. Shafts, rotors and rollers. Heavy forgings. Welds.
Castings.
Rivets and bolts. References.
The examination of non-metals and adhesive bonds. Introduction. The
testing
of concrete. The testing of wood and timber. The testing of plastics and
rubber. The testing of fibre-reiforced plastics. The testing of
ceramics. The
testing of adhesive bonds. References. Training, certification and
standards.
Tranining and certification. Standards. Further reading. Appendices.Free
damped vibrations. Time delay produced with mode conversion on internal
reflection at the lateral surface of a long cylinder. Specific acoustic
impedance for attenuated plane progressive waves. Evaluations of the
ratios of
skip distance to sound path for pipes and plates having equal
thicknesses for
given ratios of diameter to wall thickness and probe angles. Mode
conversion
caused by beam spread from a compression-wave probe located on the
curved
surface of a metal cylinder. Transit time for ultrasonic waves in
reinforced
concrete. Useful addresses. Notes on units. Index.
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Rolf Diederichs 1.August 1996, info@ndt.net